Field of the Invention
The present invention relates to a terrestrial broadcast wave reception-use antenna device for an AM band and an FM band for use in vehicles, for example, and to a component of the antenna device. In particular, the present invention relates to a configuration in which sufficient performance can be obtained even if a very short antenna element is used. Herein, the AM band is 0.522 [MHz] to 1.629 [MHz], and the FM band is 76 [MHz] to 90 [MHz].
Background Art
As an antenna element for receiving terrestrial broadcast waves, a telescopic rod antenna element having a height of approximately 1,000 [mm] has been widely used for a long time. When the telescopic rod antenna element is expanded, the telescopic rod antenna element operates as a resonant monopole antenna for approximately 0.25λ (“λ” is a wavelength of a working frequency. The same holds true hereinafter.) in the FM band. In addition, in the AM band, the telescopic rod antenna element operates as a non-resonant monopole antenna having an output impedance of approximately 10 [kΩ]. Therefore, it is known that the telescopic rod antenna element realizes a low reflection coefficient and operates at a high efficiency near 100 [%] for a receiver. Performance of the telescopic rod antenna element in the AM band and in the FM band has been practically employed as a reference as a terrestrial broadcast wave reception-use antenna for a long time, which is used for vehicles.
When the telescopic rod antenna elements are used for vehicles, there are many incidents of breakage because of their physical heights. Therefore, there is a potential demand for shortening the antenna element in the market. However, if the height of the antenna element is reduced, an accompanying impedance change of the antenna element and a mismatch loss with a cable and a receiver due to the impedance change are increased.
In response to the demand for shortening the antenna element, a rod antenna device was commercialized in the mid-1990s, which uses a rod helical antenna element having a height of approximately 200 [mm] (0.05 of an FM wavelength A). Japanese Patent Laid-Open No. 10-215122 proposes an example of the rod antenna device.
In the rod antenna device of Japanese Patent Laid-Open No. 10-215122, a monopole antenna element is connected to an amplifier unit including an AM/FM branching circuit, an FM matching circuit, an FM amplifier, an AM amplifier, and an AM/FM combining circuit. This amplifier unit performs an active impedance conversion of an impedance variation of the antenna element and reduces a mismatch loss between the antenna element and the receiver so as to compensate for performance degradation due to the shortening of the antenna element. In this way, because the performance degradation due to the shortening of the antenna element is compensated for by the amplifier unit, this type of antenna device has been widespread worldwide to date.
In the 2000s, without limiting to vehicles, types and the number of the antenna elements for wireless media mounted in mobile terminals were rapidly increased. Along with this, the demands of the market for shortening of each antenna element were further increased. As a result, in the market of the antenna for vehicles, there have been increased demands for reducing the height of the antenna element to approximately 55 [mm] (approximately 0.0125-fold of the FM band wavelength λ, for example).
However, performance degradation of the antenna element is conspicuous when the length of the antenna element is shortened as described above, and the technology of Japanese Patent Laid-Open No. 10-215122 cannot sufficiently compensate for the performance degradation by the amplifier unit. In particular, in the conventional technology, a large deterioration of a signal/noise (S/N) ratio occurs in the FM band. Therefore, no antenna device having a height of the antenna element of approximately 55 [mm] is widespread worldwide at present.